Combine garlic, rosemary, and olive oil in a bowl. Place steaks in mixture and marinate for 1 hour before cooking. On low heat, melt butter in a small saucepan. Add grated onions, minced garlic, and sugar. Using a wooden spoon stir mixture for approximately 3 minutes. Add remaining ingredients. Allow sauce to simmer 8 minutes, stirring occasionally.
Heat cast-iron with grill marks until skillet is very hot. Remove steak from marinade and pat semi dry. Place steaks into hot skillet and cook to desired temperature. Baste steak with barbecue sauce to taste.